In order to investigate how energy expenditure changes with over and underfeeding the following studies are underway. In one study, after careful calibration of weight maintenance EE, individuals undergo a series of measurements of 24 hour EE in a respiratory chamber in which they are fasting or overfed (by 200% of weight maintenance needs) a series of diets that vary in macronutrient content. This is to further investigate whether low protein diets may improve the detection of recruitment of adaptive thermogenesis. In addition, behavioral, metabolic and hormonal tests are performed to examine associated characteristics and to investigate the mechanism of the changes in EE. These individuals will also be followed up long term to look at what factors predict weigh change. In another study, after measurement of EE with over and underfeeding, and also after undergoing a series of metabolic and behavioral testing (including biopsies of muscle and fat), individuals will be admitted for 6 weeks of an inpatient dietary protocol involving underfeeding (for overweight and obese individuals) or overfeeding (for lean, obesity resistant individuals). During the inpatient study, all aspects of food intake, energy expenditure, and energy loss will be carefully measured to determine if differences in weight gain or loss can be attributed to recruitment of adaptive thermogenesis or other factors. Outpatient follow-up will continue after the inpatient phase to look at free living predictors of weight change as well.